Specialized thyristor semiconductor devices have been widely used to protect electrical equipment from line transient conditions, such as those induced by lightning strikes and power line crossings. Equipment used in data and voice communication systems is particularly sensitive to such transient conditions, although in practice many other electronic systems suffer similar sensitivities. Devices used to suppress transients are commonly referred to as Over-Voltage Protection (OVP) devices or Thyristor Surge Protection Devices (TSPD's).
OVP Thyristors have a relatively large junction area in order to withstand the typical peak currents present in a transient. The large junction area results in high junction capacitance limiting the use of OVP thyristors in some high bandwidth applications. Gas discharge tubes are more often used in high bandwidth OVP applications; an older technology having desirable low capacitance effects, but generally less reliable than solid-state devices such as thyristors.
A thyristor OVP device is generally supplied as a two terminal device for insertion between a pair of conductors. The device is often segmented into a pair of anti-parallel devices, one device for conducting a transient current of one polarity, and the other to conduct in the opposite polarity. In contrast to a common thyristor, an OVP thyristor is usually gateless, the triggering occurring via avalanche breakdown. U.S. Pat. No. 5,281,832 to Clark et al, describes an example of a bidirectional un-gated OVP thyristor.
In general, in solid-state devices, and in particular in thyristors, there have been no abrupt or quantum improvements specifically intended to lower capacitance; rather, continuous improvements have been made within a conventional design domain.